The present disclosure relates to a fuel vapor treatment system installed in an internal combustion engine.
An internal combustion engine of a vehicle may include a fuel vapor treatment system having a canister that adsorbs fuel vapor to limit emission of fuel vapor generated in the fuel tank into the atmosphere.
For example, Japanese Laid-Open Patent Publication No 2010-242723 describes a fuel vapor treatment system that includes a canister, a vapor passage which introduces fuel vapor generated in the fuel tank into the canister, and an isolation valve which opens and closes the vapor passage. The fuel vapor treatment system also includes a purge passage that introduces fuel vapor desorbed from the canister into the intake passage of the internal combustion engine, a purge valve which is located in the purge passage and adjusts the flow rate of fuel vapor, and an outside air introduction passage, which is connected to the canister and introduces outside air into the canister.
There is a limit to the amount of fuel vapor the canister can adsorb. Thus, the fuel vapor treatment system opens the purge valve to desorb fuel vapor from the canister while the engine is running. The desorbed fuel vapor is introduced into the intake passage through the purge passage and burned in the combustion chamber. Such a process is referred to as a purging process and maintains the adsorption capacity of the canister.
The fuel vapor treatment system of the '723 publication opens the isolation valve when fuel consumption or decrease in the fuel temperature generates negative pressure (for example, pressure lower than the atmospheric pressure) in the fuel tank. This returns the fuel vapor adsorbed by the canister to the fuel tank. When the pressure in the fuel tank becomes the atmospheric pressure, the isolation valve is closed to seal the fuel tank. The isolation valve is thus operated to confine fuel vapor within the fuel tank and reduce the amount of fuel vapor adsorbed by the canister. Thus, the adsorption capacity of the canister may be maintained even when the purging process is performed less frequently.
In order to confine fuel vapor within the sealed fuel tank, it may be desirable that the pressure in the fuel tank be less than the atmospheric pressure, that is, be negative pressure. Thus, the isolation valve should likely be closed when the pressure in the fuel tank becomes a predetermined negative pressure near the atmospheric pressure, instead of when the pressure in the fuel tank reaches the atmospheric pressure. However, closing the isolation valve under such a pressure condition may cause the following problem.
That is, when the purging process starts and the purge valve is opened, the negative pressure in the intake passage acts on the pressure in the canister, which is equivalent to the atmospheric pressure. Thus, the pressure in the canister (hereinafter referred to as canister pressure) may significantly decrease compared to the pressure immediately before starting the purging process. If the purge passage is clogged, the amount of fuel vapor flowing through the purge passage may be reduced, lowering the decrease amount of the canister pressure.
As such, the fuel vapor treatment system monitors the decrease amount of canister pressure after starting the purging process in order to identify whether the purge passage is clogged.
But when the isolation valve is closed at the predetermined negative pressure near the atmospheric pressure, the canister pressure immediately after closing the isolation valve is negative pressure. Thus, if a purging process is started immediately after closing the isolation valve, the decrease amount of canister pressure after starting the purging process is small since the canister pressure immediately before starting the purging process is negative pressure. This may cause unclogged purge passage to be wrongly identified as clogged.